a) Field of the Invention
The invention describes a laser resonator design that provides a laser output with an extremely high signal-to-noise (S/N) -level in each laser line of a multi-wavelength laser coupled to a single-mode fiber or to a polarization-preserving single-mode fiber, such as a multi-wavelength Ar- or ArKr-ion gas laser, and the application of this laser as light source providing illumination for a laser scanning microscope.
b) Description of the Related Art
Fundamental Gaussian Mode
For many applications it is required, that the light that emerges from a laser at a given wavelength has a lateral spatial beam profile of a pure Gaussian TEM.sub.oo -mode. This requires some technical means to suppress the propagation of higher order modes, TEM.sub.mn, at this wavelength (laser line) inside the laser cavity. Typically this is done by using intracavity mode apertures (e.g. variable iris apertures or fixed apertures such as the laser capillary), that provide sufficient loss for the higher order Gaussian modes so that for those modes the gain from the active medium does not exceed the cavity losses after one round-trip in the laser resonator (higher order Gaussian modes have larger beam diameters as compared to the fundamental Gaussian mode; see FIG. 1). Therefore, these cavity modes will not be able to reach the laser threshold and are not present in the laser output.
Cavity Modes
The propagation of the TEM.sub.mn -mode of a given laser line (wavelength .lambda.) is described by the propagation theory of Gaussian modes (A. E. Siegman, `Lasers`, University Science Books, Mill Valley, Calif., 1986) If the laser resonator is a standing wave resonator, the cavity is typically formed by two concave, or by one flat and one concave mirror, respectively, where the concave side of the mirror(s) is facing the cavity. At the reflecting surface of the concave mirror, the radius of curvature of the mirror has to be identical to that of the Gaussian wave front of the laser beam propagating inside the cavity. Only then the mode is matched to the cavity and is not changed upon reflection at the concave mirror (condition that after one cavity round trip the mode reproduces itself). The same condition holds for the other cavity mirror. From the concave mirror, the mode propagates inside the cavity until it forms a beam waist (smallest beam diameter) at the waist location either inside the cavity (e.g. confocal, semi-confocal or nearly confocal cavity) or on a flat mirror (hemi-spherical cavity). The size of the beam waist depends on the wavelength of the laser radiation and is larger for longer wavelengths. Therefore, for a multi-wavelengths laser that uses a fixed intracavity mode aperture, a given TEM.sub.mn -mode of the longer wavelength laser line is apertured more than the same mode of a shorter wavelength laser line. Typically, for multi-wavelength lasers, that aim for single-transverse mode operation (fundamental Gaussian mode TEM.sub.oo) for all laser lines, the longest wavelength laser line is "over-apertured" (i.e. for this wavelength the aperture size is smaller than the beam diameter at the aperture position) while the shortest wavelength laser line is "under-apertured" (i.e. for this wavelength the aperture size is larger than the beam diameter at the aperture position, see FIG. 2a). This results in a pure fundamental Gaussian output (TEM.sub.oo) with a somewhat lower intensity at the longest wavelength, while the shortest wavelength output is still a superposition of fundamental and higher order Gaussian modes.
Mode Competition
In a laser cavity, a mode will be able to reach the laser threshold only if the round-trip gain from the active laser medium exceeds the sum of all round-trip losses. Therefore, all modes present in a laser resonator at one laser line compete against each other to collect as much line inversion as possible. This phenomenon is known as mode competition and results in an exchange of energy between the different cavity modes over time. It can be observed as an intensity fluctuation over time in an isolated cavity mode. On the other hand, as the total amount of energy in all cavity modes for one laser line is constant (as long as no line competition is present), no fluctuation will be observed in the total line output intensity.
Single-mode Fiber Coupling
If the output of a multi-line laser as described above (pure fundamental Gaussian output at the longest wavelength, while the shortest wavelength output is still a superposition of fundamental and higher order modes) is coupled to a single-mode fiber with a cutoff wavelength (.lambda..sub.cut-off =2*.pi.*a*NA/2.405, where a and NA are the core radius and the numerical aperture of the fiber, respectively) shorter than the shortest wavelength emitted by the laser, the fiber only propagates the fundamental Gaussian mode (TEM.sub.oo) of each laser line and strips off all the higher order modes (spatial filter function of the single mode fiber). As a result, due to the separation of fundamental and higher order modes for the shortest wavelength, there can be a significant intensity fluctuation in the shortest wavelength laser line at the output of the single-mode fiber. For many applications in laser scanning microscopy, this cannot be accepted and was the driving force for this invention.